1. Field of the Invention
This invention pertains to elastomer composites including a blend of elastomers, and methods of making such composites.
2. Description of the Related Art
Numerous products of commercial significance are formed of elastomeric compositions wherein particulate filler is dispersed in any of various synthetic elastomers, natural rubber or elastomer blends. Carbon black, for example, is widely used as a reinforcing agent in natural rubber and other elastomers. It is common to produce a masterbatch, that is, a premixture of filler, elastomer and various optional additives, such as extender oil. Carbon black masterbatch is prepared with different grades of commercially available carbon black which vary both in surface area per unit weight and in structure, which describes the size and complexity of aggregates of carbon black formed by the fusion of primary carbon black particles to one another. Numerous products of commercial significance are formed of such elastomeric compositions of carbon black particulate filler dispersed in natural rubber. Such products include, for example, vehicle tires wherein different elastomeric compositions may be used for the tread portion, sidewalls, wire skim and carcass. Other products include, for example, engine mount bushings, conveyor belts, windshield wipers and the like.
There are a variety of methods for producing masterbatch. In one method, disclosed in U.S. Pat. No. 6,048,923, the contents of which are incorporated by reference herein, a continuous flow of a first fluid including an elastomer latex is fed to the mixing zone of a coagulum reactor. A continuous flow of a second fluid including a carbon black slurry is fed under pressure to the mixing zone to form a mixture with the elastomer latex. The mixing of the two fluids is sufficiently energetic to substantially completely coagulate the elastomer latex with the carbon black prior to a discharge end of the coagulum reactor. As disclosed in U.S. Pat. No. 6,929,783, the coagulum may then be fed to a dewatering extruder.
For some applications, it is desirable to employ blends of elastomers to optimize the mechanical properties of the masterbatch and/or a vulcanized rubber product of the masterbatch. Blends of elastomers may be produced by dry-mixing two polymers together. Alternatively, blends may be produced by co-coagulating a mixture of elastomer latices (see, e.g., U.S. Pat. No. 4,271,213) or by recovering a polymer blend from a mixture of an elastomer latex and a solution containing a second polymer (see, e.g., U.S. Pat. No. 5,753,742). However, none of these references disclose a wet masterbatch method of combining a filler with two or more elastomers. Rather, the polymer blends produced by these methods may be dry mixed with filler to form a masterbatch. However, dry mixing can reduce polymer molecular weight and degrade polymer properties.
U.S. Pat. No. 6,521,691 discloses a wet masterbatch method in which two polymer emulsions and a filler slurry may be combined and then coagulated to form a masterbatch. Similarly, U.S. Pat. No. 6,800,126 discloses that carbon black aggregates may be blended with an oil and a low-molecular weight elastomer latex to form a preblend, which preblend may be combined with an emulsion of a diene-based elastomer. A composite of the elastomer and the carbon black is then recovered from the mixture. U.S. Pat. No. 4,578,411 discloses a method in which an elastomer latex, an elastomer solution, and a filler are combined, following which a composite of the two elastomers with the filler is recovered. However, all of these references disclose the combination of the filler with the elastomer and the coagulation of the latex as separate processing steps. Between these two process stages, the filler can flocculate in the emulsion to the detriment of its even distribution in the final composite.
U.S. Pat. No. 6,048,923 discloses a wet masterbatch method in which coagulation ensues upon introduction of a filler slurry to an elastomer latex, resulting in superior dispersion of filler within the elastomer composite. No coagulant is required to form this “wet mix” elastomer composite. The '923 patent discloses that blends of elastomer latices may be combined with the filler slurry to form a coagulum. However, because natural rubber latex is essentially a biological material, it contains a number of dissolved substances such as magnesium, potassium, phosphate, and sulfate ions, fatty acids, amino acids, carbohydrates, peptides, etc. The high ionic strength of the aqueous carrier can cause synthetic latices with which natural rubber latex is combined to coagulate before the filler slurry is introduced to the elastomer latex blend. Such synthetic latices are said to be incompatible with natural rubber latex. While it is possible to render the two latices compatible by adding surfactant to either the synthetic or the natural latex, the additional surfactant makes the latex more difficult to coagulate and increases the concentration of surfactant in the final rubber product. Thus, it is desirable to have a method of producing elastomer composite masterbatches incorporating a blend of materials that is not susceptible to the problem of early coagulation but which still results in a homogeneous dispersion of filler within the elastomer.